1. Field of the Invention
The present invention relates to a red surface emitting laser element and an image forming device and an image display apparatus incorporating the red surface emitting laser element.
2. Description of the Related Art
A. Usefulness of Red Surface Emitting Laser Element
A surface emitting laser element (in particular, a surface emitting laser of a vertical cavity type is called a vertical cavity surface emitting laser (VCSEL)) can output light in a direction perpendicular to the surface direction of the semiconductor substrate and can relatively easily be formed as a two dimensional array.
When the element is formed as a two dimensional array, parallel processing is realized by multiple beams emitted therefrom. Thus, application of this two dimensional array technology to various industrial usages is desired to achieve a higher density and a higher speed.
For example, the surface emitting laser array may be used as an exposure light source of an electrophotographic printer so that the printing rate can be increased by parallel processing of the printing step using multiple beams.
A surface emitting laser currently put into practice is an element that outputs a laser beam in the infrared region (wavelength λ=0.75 μm to 1 μm). If the oscillation wavelength is further shortened, the beam diameter can be further reduced and an image with a higher resolution can be obtained.
A red surface emitting laser element outputs light having a wavelength (about 0.6 μm to about 0.73 μm) shorter than that in the infrared region. Moreover, at this wavelength, the sensitivity of amorphous silicon applicable to a photosensitive drum of an electrophotographic printer is very high.
Thus, red surface emitting lasers are now desired to be put into practical for use in photosensitive drums composed of amorphous silicon to achieve higher speed, higher resolution image printing.
The effect brought about by the combination of an increase in resolution by shorter wavelength and multi-beam parallel processing is significantly large. This combination is expected to make contributions in various fields including electrophotographic printers and image display apparatuses such as laser displays.
B. Basic Structure of Red Surface Emitting Laser
In order to generate light having a wavelength in the red region, a semiconductor material, AlGaInP is typically used. The lattice of this material matches with the lattice of GaAs, which is the material constituting the deposition substrate, and the bandgap energy can be controlled by varying the compositional ratios of aluminum and gallium.
In order to generate laser oscillation, a threshold current or higher must be injected to the laser element. Current injection allows carriers, such as electrons or holes, to be injected to the active layer, and the carriers are eventually converted to light as they undergo radiative recombination.
C. Specific Examples of Related Art
A red surface emitting laser is formed by interposing a resonator region including an AlGaInP active layer between multilayered reflectors composed of a different semiconductor material, AlGaAs. A GaAs substrate whose lattice matches with those of the active layer and the multilayered reflectors is used as the substrate.
In 1995, a group led by Crawford of Sandia National Laboratories disclosed the element structure of a 1-wavelength resonator structure (see M. H. Crawford et al., IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 7, No. 7 (1995), 724, hereinafter referred to as “Crawford reference”).
This one wave resonator structure has the most widely used cavity length in surface emitting lasers that output infrared emission. In red surface emitting lasers, the 1 wavelength cavity length is about 200 nm (in the case where the wavelength is 680 nm) in terms of layer thickness.
In particular, an active layer having a multiquantum well structure 40 nm to 50 nm in thickness is disposed in the central region of the 1-wavelength cavity length. A p-type AlGaInP layer and an n-type AlGaInP layer which each function as a spacer layer and have a thickness of 80 nm or less are disposed on both sides of the active layer.
In some cases, an undoped spacer layer is disposed between the active layer and the doped p-type (or n-type) spacer layer. In such cases, the thickness of the p-type (or n-type) AlGaInP spacer layer is about 50 nm.
In Crawford reference, the thickness of the p-type or n-type AlGaInP layer is about 50 nm.
Crawford reference also teaches that the maximum light output power at a 675 nm mode is 2.8 mW (20° C.) from the element with 15 μm φ oxide aperture.